The invention relates to a tensioning and damping element consisting of an elastomeric material for chain drives, especially a tensioning and damping element for the secondary chain drive of a motorcycle exhibiting temporary alteration of the distance between the centers of the sprocket wheels.
In motorcycles having a secondary chain drive and a rear wheel swinging arm, there is the problem of the temporary alteration of the distance between the centers of the sprocket wheels during the swinging movement away from the middle position of the swinging arm, which is used as the position for adjusting the slack of the chain and the distance between the centers of the sprocket wheels, in the phases of the upper, deflected end position of the swinging arm and the lower, rebound end position of the swinging arm. Upon deflection and rebounding of the swinging arm, therefore, the distance between the centers of the sprocket wheels is also always reduced, with the effect that the chain slack in the non-loaded strand becomes greater. The cause of this is the position of the sprocket wheel at the gearbox output which is determined by the design and which does not coincide with the pivot of the rear wheel swinging arm but lies in front of the pivot in the direction of travel.
The temporary alteration of the distance between the centers is combined at the same time with an enforced rotation of the rear wheel, since the rear wheel is rolling along the roadway when the distance of the center point of the wheel from the sprocket wheel at the gearbox output changes. The same rotational movement as the rear wheel is performed, therefore, by the sprocket wheel on the rear wheel and, at a suitable gear ratio, also by the sprocket wheel at the gearbox output. The temporary alteration of the distance between the centers and the enforced rotation of the rear wheel by the rolling movement during deflection and rebounding exacerbate other and impair the driving stability of the motorcycle.
When there is a large amount of slack in the non-loaded strand, oscillations and shocks furthermore occur in the non-loaded strand, which have an adverse effect on the driving characteristics of the motorcycle. To avoid that disadvantage, therefore, a spring-loaded chain tensioning device directed towards the non-loaded strand is usually provided.
In the transition phases from acceleration to braking or during gear-changing, there will also be phases in which the loaded strand also is not loaded. One also speaks in this context of xe2x80x9cdouble non-loaded strand phasesxe2x80x9d in which also the loaded strand briefly runs under no load. Those phases are especially dangerous, since they may cause the circulating, temporarily overlong, chain which is loaded only by centrifugal force to ride up on the sprocket wheel at the gearbox end, which may result in indifferent changing of the gear ratio of the secondary chain drive and/or in skipping of the chain.
The alteration of the distance between the centers, the enforced rotation of the rear wheel and the double non-loaded strand phases result in indifferent phases of the vertical dynamics of the vehicle which are affected by springing, damping and tire resilience and in indifferent phases of the longitudinal dynamics (shifting of the center of gravity and pitching motion) of the motorcycle. When cornering with changing load caused by braking or acceleration and when there is a large amount of slack in the non-loaded strand in the lower, rebound end position of the swinging arm, the contact surface of the tires may be so greatly reduced that it is no longer possible to control the motorcycle and a fall is inevitable.
A tensioning and damping element for chain drives is known from DD 275 166 A3. In that specification, the element is an axleless ring of elastic material which mates with the chain and which is elastically deformable between the shape of a concentric ring and the shape of a Cassini curve having an ellipse-like configuration. That tensioning and damping element is arranged between strands of the chain drive and engages therein by its toothed rim, automatically securing its position in the chain drive and tensioning diametrically.
A tensioning and damping element is also known from DE 43 17 033 C1, which will be described in more detail below with reference to FIGS. 2 and 3 using, as an example, a secondary chain drive of a motorcycle exhibiting temporary alteration of the distance between the centers of the sprocket wheels.
The chain drive shown in FIGS. 2 and 3 has a sprocket wheel 1 at the gearbox end, a rear wheel sprocket wheel 2 and a circulating chain consisting of a loaded strand 5 and a non-loaded strand 6. Arranged between the non-loaded strand and the loaded strand is a tensioning and damping element 7 which assumes an ellipse-like shape in this stressed, installed state. That tensioning and damping element comprises an elastically deformable ring part and a toothed rim arranged at the periphery thereof, the ring part being circular in the stress-free, uninstalled state. In the installed state, the toothed rim engages with the non-loaded strand and the loaded strand, whereby a tensioning force caused by the ellipse-like deformation of the ring part is transmitted to the two strands.
The rear wheel sprocket wheel 2 is fastened to one end of a rear wheel swinging arm 4 which, at the other end, can be pivoted about a swinging arm pivot 3 from the middle position shown in FIG. 2 into a deflected and a rebound end position of the swing. In FIG. 3, the deflected end position of the swinging arm is shown.
Owing to the fact that the swinging arm pivot 3 and the pivot of the sprocket wheel 1 at the gearbox end do not coincide, temporary alteration of the distance between the centers of the two sprocket wheels 1, 2 occurs upon movement of the rear wheel swinging arm 4. In the middle position shown in FIG. 2, a maximum distance between centers amax is produced and, in the two end positions, see FIG. 3, a minimum distance amin. As a result of the distance between the centers being reduced, the chain slack in the non-loaded strand 6 increases.
The biased, installed tensioning and damping element always strives to attain its stress-free state in which it has a circular shape. The tensioning and damping element is so dimensioned that it never assumes its stress-free state in the installed state. That ensures that, even in the deflected end position shown in FIG. 3, in which greater chain slack occurs, a tensioning force will still be transmitted to the two strands by the tensioning and damping element. The tensioning and damping element therefore changes its shape in dependence upon the distance between the centers of the two sprocket wheels 1, 2, as can be seen especially from FIGS. 2 and 3. In the state shown in FIG. 2, the distance between the minor axis extremities N1, N2 of the tensioning and damping element deformed into an ellipse-like shape is smaller than in the state shown in FIG. 3. In addition to compensating for chain slack, the tensioning and damping element also damps deflections of the chain which may arise, for example, as a result of polygonal and over-running shocks and the effects of gravity and/or centrifugal force.
The tensioning force and the damping increase with increasing ellipse-like deformation of the tensioning and damping element as a result of dynamically elastic and plastic deformation parameters. In addition, the tensioning force is at the same time increased by static deformation parameters.
That means, however, that during driving operation, the tensioning and damping element 7 is dynamically loaded to a greater extent in the phases in which the rear wheel swinging arm 4 is in the middle position shown in FIG. 2, and in which the amount of chain slack is consequently smaller, than it is in the deflected end position of the rear wheel swinging arm shown in FIG. 3.
During operation of the chain drive, the tensioning and damping element rotates therewith owing to its engagement with the loaded and the non-loaded strand. In the process, elastic and plastic deformation occurs continuously in the ring part, the radially outside and inside regions of the ring part, in particular, being especially loaded. The deformation and recovery from deformation ultimately manifests itself in heat loss of the tensioning and damping element. Since, by reason of the material, the heat loss must not exceed a certain value in order to obtain sufficient tensioning force and damping, the maximum permissible rotation speed of the tensioning and damping element is predetermined.
Furthermore, large recovery distances and high recovery speeds of the elastic and plastic deformation components lead to more rapid fatigue of the material.
The disadvantage of the known tensioning and damping element for use in secondary chain drives on motorcycles having a rear wheel swinging arm is accordingly the fact that it is operated in the most frequently occurring middle position phase of the swinging arm, having the then correspondingly smaller amount of chain slack, in the range of its greatest ellipse-like deformation, and that, consequently, greater heat loss and material fatigue occurs as a result of the rotation.
As a result, in addition, the maximum permissible chain speed of the secondary chain drive and the maximum permissible speed of travel of the motorcycle are limited.
The problem underlying the invention is therefore to develop the tensioning and damping element in such a manner that it produces less lost heat in operation and is subject to slower material fatigue.
According to the invention, the ring part has a cross-section that, with increasing flection, exhibits a decreasing flexural resistance owing to its deformation, and, in the maximally deformed state caused by operation, the height to width ratio of the cross-section is at least 4% smaller than in the stress-free state.
The height to width ratio has a direct effect on the flexural resistance which opposes increasing deformation of the ring part. Thus, at a smaller height to width ratio, the flexural resistance also decreases.
That effect is especially advantageous in the case of a tensioning and damping element for a secondary chain drive of a motorcycle exhibiting temporary alteration of the distance between the centers of the sprocket wheels, since it is precisely in the middle position phase of the rear wheel swinging arm, in which the non-loaded strand has its smallest amount of slack in any case, that is to say when deflections and shocks are minimal, and in which the tensioning and damping element exhibits its greatest elliptical deformation, that the alteration of the cross-sectional profile results in a less flexurally resistant cross-section, so that ultimately less heat loss is produced and material fatigue is minimized.
In the case of the known ring part, in which the cross-section essentially does not change upon deformation, during rotation of the ring part, alternating uniaxial bending strains occur with stress concentrations in the peripheral fibers at the inside and the outside of the ring during passage through the regions of the major and minor axis extremities of the ring part deformed into an ellipse-like shape.
In the case of the ring part according to the invention, on the other hand, under the bending strain, a multi-axial stress distribution occurs with automatic deformation of the cross-sectional profile subjected to the bending strain to a less flexurally resistant cross-sectional profile. The special feature existing here is that the cross-sectional profile subjected to bending load yields by changing shape in several axial directions, specifically, however, into a shape having a lower flexural resistance, by virtue of the fact that the peripheral fibers, which are subject to greater load, orient themselves closer to the neutral bending axis as a result of the height reduction. The stresses caused by the rolling of the tensioning and damping element, which occur in the cross-sectional plane of the ring profile upon passage through the regions of the major and minor axis extremities (H1, H2, N1, N2) of the tensioning and damping element, are coupled with transverse expansion of the cross-section subjected to bending strain. That causes a kind of opening movement or a widening in the x axis and a constriction in the y axis of the cross-sectional profile. The cross-sectional profile existing after that spreading movement or widening and constriction has a lower flexural resistance than before. The deformation of the cross-sectional profile increases steadily with the deformation of the tensioning and damping element from its annular shape to the ellipse-like shape. With increasing deformation, therefore, the tensioning and damping element changes in a self-regulating manner to a state in which the flexural resistance or the tensioning force is reduced.
The tensioning and damping element accordingly has the advantage that, despite relatively great deformation in certain phases of the chain drive, especially during acceleration, braking and driving phases of a motorcycle in a relatively high speed range, associated with the middle position phase of the rear wheel swinging arm which then exists, it is subject to lower static, dynamic and thermal load, with the result that the maximum permissible chain speed can be increased.
Conversely, the tensioning and damping element has the advantage that, in the case of relatively slight deformation and a large amount of chain slack, it still tensions the chain sufficiently tightly. Such a situation arises in the case of motorcycles having a rear wheel swinging arm especially in the double non-loaded strand phase of the chain drive which can occur, for example, when shifting to a lower gear while simultaneously braking, double-declutching or engaging the clutch after changing gear combined with deflection and rebounding of the rear wheel swinging arm. Further embodiments of the tensioning and damping element will be explained in more detail using the following description of a number of illustrative embodiments and the drawings.